1. Field of the Invention
This invention relates to aircraft gas turbine engines and, more particularly, to FLADE engines.
2. Description of Related Art
High performance variable cycle gas turbine engines are being designed because of their unique ability to operate efficiently at various thrust settings and flight speeds both subsonic and supersonic. An important feature of the variable cycle gas turbine engine which contributes to its high performance is its capability of maintaining a substantially constant inlet airflow as its thrust is varied. This feature leads to important performance advantages under less than full power engine settings or maximum thrust conditions, such as during subsonic cruise.
One particular type of variable cycle engine called a FLADE engine (FLADE being an acronym for “fan on blade”) is characterized by an outer fan driven by a radially inner fan and discharging its FLADE air into an outer fan duct which is generally co-annular with and circumscribes an inner fan duct circumscribing the inner fan. One such engine, disclosed in U.S. Pat. No. 4,043,121, entitled “Two Spool Variable Cycle Engine”, by Thomas et al., provides a FLADE fan and outer fan duct within which variable guide vanes control the cycle variability by controlling the amount of air passing through the FLADE outer fan duct. Other high performance aircraft variable cycle gas turbine FLADE engines capable of maintaining an essentially constant inlet airflow over a relatively wide range of thrust at a given set of subsonic flight ambient conditions such as altitude and flight Mach No. in order to avoid spillage drag and to do so over a range of flight conditions have been studied. This capability is particularly needed for subsonic part power engine operating conditions. Examples of these are disclosed in U.S. Pat. No. 5,404,713, entitled “Spillage Drag and Infrared Reducing Flade Engine”, U.S. Pat. No. 5,402,963, entitled “Acoustically Shielded Exhaust System for High Thrust Jet Engines”, U.S. Pat. No. 5,261,227, entitled “Variable Specific Thrust Turbofan Engine”, and European Patent No. EP0,567,277, entitled “Bypass Injector Valve For Variable Cycle Aircraft Engines”. A FLADE aircraft gas turbine engine with counter-rotatable fans is disclosed in U.S. patent application Ser. No. (10/647,881), entitled “FLADE GAS TURBINE ENGINE WITH COUNTER-ROTATABLE FANS”.
FLADE engines have the fan blade attached to one of the front fans. This can lead to low pressure spool designs that are compromised because of the limitations in rotor speeds and increased stresses caused by the FLADE blade attachment and location. The front fan mounted FLADE fan blades also are difficult to adapt to present engines or engine designs. It would be very expensive to adapt an existing engine to test a front fan mounted FLADE fan. It would be difficult to demonstrate some of the system benefits offered by a FLADE engine concept at a reasonable cost relative to that of a new low pressure system or defined around an existing core engine.
It is highly desirable to have a FLADE engine that allows a low pressure spool design that is uncompromised because of limitations in rotor speeds and increased stresses caused by the FLADE blade attachment and location. It is highly desirable to have an engine in which FLADE fan blades are not difficult to adapt to present engines or engine designs and that would not be very expensive to adapt to an existing engine to test as compared to a front fan mounted FLADE fan. It is also desirable to be able to demonstrate some of the system benefits offered by a FLADE engine concept without a great deal of difficulty and at a reasonable cost relative to that of a new low pressure system or defined around an existing core engine.